[sector24]

The discount is not a penalty on globals at all. If you're casting Mother Oak for 40 instead of 50, you can just choose to add 10 gems on top of the base cost and it's the same as if the magic site isn't there. But as thejeff pointed out, you can cast it for 40 and recoup your nature gems more quickly if you know it's going to get dispelled anyway. So while the magic site isn't a bonus, it does provide potential flexibility.